Is there a measurement overlap between depressive symptoms and quality of life?

Background: Previous studies found that depression is associated with a broad impairment in quality of life (QOL). This finding might be associated to a measurement overlap. Methods: The objective of this study was to verify whether the items of the World Health Organization Quality of Life Instrument-Abbreviated version (WHOQOL-BREF), a measure of generic QOL, are invariant among patients having a current major depressive episode who come from primary care services. We investigated data from primary care services from the 6 countries (Australia, Brazil, Israel, Russia, Spain, and the United States) involved in the baseline sample of the Longitudinal Investigation of Depression Outcomes. The Rasch model was used to analyze items exhibiting differential item functioning (DIF) as a way of assessing invariance in relation to a depression factor defined by the diagnosis of depression using the Composite International Diagnostic Interview. In addition, the Center for Epidemiological Studies-Depression Scale (CES-D) score was correlated with the item and domain scores of the WHOQOL-BREF using the Pearson coefficient. Results: The sample consisted of 2359 subjects, of which 1193 had a confirmed diagnosis of a current major depressive episode. Of the 26 items of the WHOQOL-BREF, I I showed DIF due to the depression factor, and the physical domain presented more items displaying DIF. All Pearson coefficients between the WHOQOL-BREF item and domain scores and the CES-D score were weak and moderate (r = -0.13 to r = 0.43). Conclusions: Our findings indicate that most WHOQOL-BREF items do not exhibit DIF for a current major depressive episode and the variance associated with depression in this generic QOL measure is restricted to some facets of this construct. Thus, we recommend this restricted adjustment for depression in future analyses of this measure. Furthermore, our study indicates that researchers must measure QOL regardless of depression severity. (C) 2009 Elsevier Inc. All rights reserved.